Performance comparison of wavefront reconstruction and control algorithms for Extremely Large Telescopes (original) (raw)
Related papers
Journal of the Optical Society of America A, 2009
The performances of various estimators for wavefront sensing applications such as adaptive optics (AO) are compared. Analytical expressions for the bias and variance terms in the mean squared error (MSE) are derived for the minimum-norm maximum likelihood (MNML) and the maximum a posteriori (MAP) reconstructors. The MAP estimator is analytically demonstrated to yield an optimal trade-off that reduces the MSE, hence leading to a better Strehl ratio. The implications for AO applications are quantified thanks to simulations on 8-m-and 42-m-class telescopes. We show that the MAP estimator can achieve twice as low MSE as MNML methods do. Large AO systems can thus benefit from the high quality of MAP reconstruction in O͑n͒ operations, thanks to the fast fractal iterative method (FrIM) algorithm (Thiébaut and Tallon, submitted to J. Opt. Soc. Am. A).
Performance of the Fourier transform reconstructor for the European Extremely Large Telescope
Adaptive Optics Systems, 2008
The forthcoming Extremely Large Telescopes, and the new generation of Extreme Adaptive Optics systems, carry on a boost in the number of actuators that makes the real-time correction of the atmospheric aberration computationally challenging. It is necessary to study new algorithms for performing Adaptive Optics at the required speed. Among the last generation algorithms that are being studied, the Fourier Transform Reconstructor (FTR) appears as a promising candidate. Its feasibility to be used for Single-Conjugate Adaptive Optics has been extensively proved by L. Poyneer [1]. As part of the activities supported by the ELT Design Study (European Community's Framework Programme 6) we have studied the performance of this algorithm applied to the case of the European ELT, in two different cases: singleconjugate and ground-layer adaptive optics and we are studying different approaches to apply it to the more complex multi-conjugate case. The algorithm has been tested on ESO´s OCTOPUS software, which simulates the atmosphere, the deformable mirror, the sensor and the closed-loop control. The performance has been compared with other algorithms as well as their response in the presence of noise and with various atmospheric conditions. The good results on performance and robustness, and the possibility of parallelizing the algorithm (shown by Rodríguez-Ramos and Marechal-Hernández) make it an excellent alternative to the typically used Matrix-Vector Multiply algorithm.
Fast minimum variance wavefront reconstruction for extremely large telescopes
Journal of the Optical Society of America A, 2010
We present a new algorithm, FRiM (FRactal Iterative Method), aiming at the reconstruction of the optical wavefront from measurements provided by a wavefront sensor. As our application is adaptive optics on extremely large telescopes, our algorithm was designed with speed and best quality in mind. The latter is achieved thanks to a regularization which enforces prior statistics. To solve the regularized problem, we use the conjugate gradient method which takes advantage of the sparsity of the wavefront sensor model matrix and avoids the storage and inversion of a huge matrix. The prior covariance matrix is however non-sparse and we derive a fractal approximation to the Karhunen-Loève basis thanks to which the regularization by Kolmogorov statistics can be computed in O(N) operations, N being the number of phase samples to estimate. Finally, we propose an effective preconditioning which also scales as O(N) and yields the solution in 5-10 conjugate gradient iterations for any N. The resulting algorithm is therefore O(N). As an example, for a 128 × 128 Shack-Hartmann wavefront sensor, FRiM appears to be more than 100 times faster than the classical vector-matrix multiplication method.
New challenges for adaptive optics: extremely large telescopes
Monthly Notices of the Royal Astronomical Society, 2000
The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is achieved at 0.5 mm with a limiting magnitude of the AO guide source near R magnitude , 10Y at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 arcsec at 0.5 mm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faint R , 22 NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible.
Adaptive optics for Extremely Large Telescopes
Proceedings of the International Astronomical Union, 2005
Adaptive Optics (AO) will be essential for accomplishing many, if not most, of the science objectives currently planned for Extremely Large Telescopes including GMT, OWL, and TMT. AO will be needed to support a range of instrumentation, including near infrared (IR) imagers and spectrometers, mid IR imagers and spectrometers, "planet finding" instrumentation and wide-field optical spectrographs. Multiple advanced AO systems, utilizing the full range of concepts currently under development, will need to be combined into an integrated architecture to meet a broad range of requirements for field-of-view, spatial resolution and spectral bandpass. In this paper, we describe several of the possible options for these systems and outline the range of issues, trade studies and component development activities which must be addressed. Some of these challenges include very high-order, large-stroke wavefront correction, tip-tilt sensing with faint natural guide stars to maximize sky coverage, laser guide star wavefront sensing on a very large aperture and achieving extremely high contrast ratios for the detection of extra-solar planet and other faint companions of nearby bright stars.
Adaptive optics simulations for the European Extremely Large Telescope
SPIE Proceedings, 2008
Simulations of adaptive optics (AO) for the European extremely large telescope (EELT) are presented. For Shack-Hartmann wavefront sensors for the laser guide star (LGS) based systems, the simulations show that without the Rayleigh fratricide effect, central projection of the laser is preferable to side projection, the correlation or matched filter centroiding algorithms offer superior performance to a traditional center-of-gravity approach, the optimum sampling of the detector is approximately 1.5 pixels per FWHM of the non-elongated spot, and that at least 10×10 pixels are required. The required number of photo-detection events from the LGS per frame per subaperture is of the order of 1000. Correction of segmentation errors with a Shack-Hartmann wavefront sensor (WFS) has also been investigated; atmospheric turbulence dominates these segmentation errors. The pyramid WFS is also simulated for the EELT, showing that modulation of the pyramid will be necessary.
Results of a System Study for the ESO Very Large Telescope Adaptive Optics
1994
lbis paper outlines the key results of the Very Large Telescope (VLT) Adaptive Optics System Study perfonned by MMS/UTOS under an ESO contract A conceptual design was developed based entirely on available and demonstrated technologies. Key subsystems included a 250 actuator continuous facesheet Defonnable Mirror, an intensified Shack-Hartmann wavefront sensor and a DSP-based fast processor utilizing a parallel architecture. The
SPIE Proceedings, 2011
The Large Binocular Telescope (LBT) is a unique telescope featuring two co-mounted optical trains with 8.4m primary mirrors. The telescope Adaptive Optics (AO) system uses two innovative key components, namely an adaptive secondary mirror with 672 actuators and a high-order pyramid wave-front sensor. During the on-sky commissioning such a system reached performances never achieved before on large ground-based optical telescopes. Images with 40mas resolution and Strehl Ratios higher than 80% have been acquired in H band (1.6 m). Such images showed a contrast as high as 10-4. Based on these results, we compare the performances offered by a Natural Guide Star (NGS) system upgraded with the state-of-the-art technology and those delivered by existing Laser Guide Star (LGS) systems. The comparison, in terms of sky coverage and performances, suggests rethinking the current role ascribed to NGS and LGS in the next generation of AO systems for the 8-10 meter class telescopes and Extremely Large Telescopes (ELTs).
Adaptive Optics for Extremely Large Telescopes 4 – Conference Proceedings , 1 ( 1 )
2016
In this paper, we will present new compression algorithms to determine optimal layer heights and turbulence weights for the tomographic reconstruction in wide field AO systems. Among other approaches, a new compression method based on discrete optimization of collecting atmospheric layers to subgroups is discussed. Furthermore, studies of the influence of layer heights and cn-profiles on the reconstruction quality for di erent reconstruction algorithms and atmospheric profiles will be shown. Our comparison suggests that reconstructions on fewer atmospheric layers yield comparable quality with lower computational e ort, if an appropriate compression algorithm is used. The numerical results were obtained on the ESO end-to-end simulation tool OCTOPUS.